The phosgenation of aliphatic or aromatic amines for the preparation of isocyanates can be carried out with particular advantage in the gas phase. Such processes are well established in principle in the art, such as described in European Patent Publication No. FP 0289840, the entire contents of which are incorporated herein by reference, and such processes have since become established industrially.
International Patent Publication No. WO 97/24320, the entire contents of which are incorporated herein by reference, generally describes a process for producing isocyanates by reacting the corresponding (aromatic) amines with phosgene using recycled chlorine, in which process phosgene produced from specially purified chlorine from a special electrochemical cell and a non-specifically defined carbon monoxide excess is passed directly into a phosgeniation reactor. No concentration data are provided. No description is provided of the starting products or of the resulting secondary components in the phosgene or of the effect on the quality of the isocyanates produced. Nor are any details provided of the phosgenation process, whether it is conducted with or without a solvent, or of the temperature range over which the isocyanate production is carried out.
Phosgenation on the industrial scale is typically carried out using phosgene prepared in a phosgene generator over a catalyst of chlorine and carbon monoxide. Before being supplied to the phosgenation, the phosgene from the generator is first supplied to a purifying stage where it is separated into a waste gas stream and a phosgene stream, preferably by condensing out phosgene or by distillation. Prior to the actual reaction with the amine, the phosgene stream is then also usually mixed with recycled phosgene, such as disclosed in International Patent Publication No. WO 2004/037718, the entire contents of which are incorporated herein by reference.
Phosgene used in accordance with the prior art possesses residual CO gas levels, owing to the aftertreatment steps, of less than 0.05% by weight.
Following the reaction of amine and phosgene, the resultant isocyanate is purified conventionally by distillation to remove low-boiling and high-boiling by-products. In many instances it is problematic if the purified isocyanates are coloured or if subsequent modification steps such as the prepolymerization, biuretization or trimerization, for example, are accompanied by unwanted side reactions which again, ultimately, impact adversely on the colouring of the polyisocyanates obtained.
Side reactions of this kind are often triggered by very low concentrations of compounds which are chlorinated or include chlorine in a hydrolyzable form. In principle it is possible to remove those compounds which contribute to the hydrolyzable chlorine content (HC compounds and HC content) or to the total chlorine content of the isocyanates, but from a production standpoint it is undesirable to do so, since an additional purifying step of this kind increases the production costs through an increased use of energy and/or loss of yield owing to thermal loading.